| In consideration of the recent energy crisis and its associated environmental problems,ammonia(NH3),as an alternative carbonless fuel,not only can alleviate the energy crisis to a certain extent but also conform to the rising global decarbonization tendency,exhibiting broad application scenarios in the future.This dissertation proposed a plasma coupling with electrocatalysis strategy for scalable ammonia synthesis based on an unconventional path of useing plasma to activate inert N2 into more accessible NOx,followed by enhanced electroreduction of NOx-into NH3(p NOe NO2-RR).This strategy can efficiently overcome the drawbacks of high energy consumption,complex by-products in plasma and low yield rate,low selectivity in NRR.Herein,systematic investigations were carried out experimentally and theoretically,offering comprehensive insights on novel ammonia synthesis technology and facilitating the development of small-scale,distributed NH3 synthesis process in the aspect of fundamental study.Furthermore,the ammonia-urea synthesis experiments were performed based on the plasma coupling with electrocatalysis strategy,paving the way for the expansion of application scenarios.The main contents and conclusions of this dissertation are summarized as below:(1)Analysis on nitrogen fixation characteristics of air jet plasma.The fluid disturbance in plasma reactor was analyzed by CFD simulation;COMSOL simulation analysis was used to capture the spatiotemporal evolution of particles;ZDPlas Kin was adopted to obtain the possible reaction pathways of particles,showing that NO2 was mainly oxidized by NO.NO can be produced by direct interaction between N and O2(X)/O2.N is mainly produced via the reaction between NO and O(1D)radicals and O mainly derives from the reaction of O(1D)radicals with O/O2.The OES analysis was also conmnducted to give detailed information that the NO mianly is generated by the reaction of metastable N2 or electron excited state N2 with ground state/excited state O.MRU gas analyzer comfirmed that the main gas product is NO with the an avarage concentration over 12000 ppm,and IC detection comfirmed the main anion produced in electrolyte was NO2-with a formation rate of 23.6 mmol L-1 h-1 during air discharge.(2)The plasma coupling with electrocatalysis strategy for ammonia synthesis was establish and the step-by-step system was constructed.The effects of various influencing factors on ammonia synthesis performance were explored and found that the discharge distance,N2/O2 ratio,applied potential had significant effects on ammonia synthesis.The highest ammonia yield(3.14 mg h-1 cm-2)could be obtained at-0.53 V at air discharge with a distance of 1 cm,a supply voltage of 10 k V,and a flow rate of 6L min-1.The multiphysics simulation of the gas-liquid reaction was carried out by COMSOL and the temperature distribution,velocity distribution,concentration distribution of main substances,and reduced electric field,and electron energy distribution during discharge were semi-quantitatively analyzed.Evalution of system energy consumption was performed and the overall energy consumption was calculated to be 3.18 MJ mol NH3-1(plasma: 2.4 MJ mol NH3-1;electrocatalysis: 0.78 MJ mol NH3-1).(3)The intrgrated system with tandem electroreduction process was further developed.The integrated system was constructed by design optimization to solve the problems(unable to run continuously;insufficient use of NOx)in step-by-step system.A tandem-electrocatalysis process was adopted to further improve the usage of NOx and coversion efficiency of ammonia.The continuous process achieved an ammonia yield of 4.6 mg h-1 cm-2@-0.53 V,realizing a 48.4% enhancement of the step-by-step process.Furthermore,the intrgrated system with tandem electroreduction process obtained an ammonia yield of 7.3 mg h-1 cm-2,which is 2.4 times of the step-by-step system and 1.6 times of the intrgrated system with single electroreduction process.The15 N isotope labeling experiments were determained by 1H NMR detection to confirm the source of N.Consistent with the standard sample,the spectra of reacted solution exhibited two and three characteristic peaks indexed to 15NH4+ and 14NH4+,confirming the N originated from the feeding nitrogen and excluding the influence of the surroundings.The plasma tandem-electrocatalysis strategy thus open a new avenue for yield elevation.Also,the overall energy consumption was evaluated and the optimized value was calculated to be 2.40 MJ mol NH3-1(plasma: 1.41 MJ mol NH3-1;electrocatalysis:0.99 MJ mol NH3-1),which reduced 24% compared to the step-by-step system.(4)Defective N-Mo S2 electrocatalysts design and microscopic nitrogen fixation mechanism analysis.Hydrothermal combined with plasma engraving were adopted for defective N-Mo S2 preparation,achieving an efficient conversion of NO2-→NH3.Comprehensive characteristics jointly confirmed the phase transformation,N doping,and S vacancy generation.Compared with pristine Mo S2,the defective N-Mo S2 realized a 19.2 enhanced ammonia yield rate.DFT calculations were conducted to elucidate the superiority of defective N-Mo S2.Results demonstrated the advantage of1T-Mo S2 in NO2-adsorption and electron transfer characteristics.What’s more,the Gibbs free energy calculations of e NO2-RR and HER were perfromed on different NO2-and H+ adsorption sites of N-Mo S2,showing that Mo sites with N doping and S vacancies possess lower energy barrier in RDS(0.43 e V;0.68 e V)and stronger inhibition effect on HER.This explained the underlying origination of elevated e NO2-RR performance.,The ICP engraving thus provided a strategy for efficient improvement of e NO2-RR electrocatalytic performance.The microscopic mechanism analysis of N-Mo S2 also provided theorical basis of e NO2-RR.(5)Exploratory study on the plasma coupling with flow cell for ammoniaurea synthesis.To further expand the application scenarios of coupling process and provide basis for complex products synthesis experimentally and theoretically,the system of plasma coupling with flow cell was constructed for the ammonia-urea synthesis and the microscopic mechanism was analyzed in depth.A nickel-cobalt bimetallic catalyst(Ni Co2O4)was prepared by hydrothermal method and a remarkable yield rate of ammonia/urea was achieved(0.3557 mmol h-1/0.0961 mmol h-1).DFT calculations showed that Ni Co2O4 possess strong binding ability on Ni-O/Co-O bonds.The Gibbs free energy analysis of the ammonia/urea synthesis path indicated that Ni Co2O4 realized a lowest RDS barrier(ΔG = 0.66 e V/ΔG = 0.20 e V)in the dual synthesis pathway and therefore revealing the instinct reason of optimized activity in C-N coupling reaction. |
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